This invention relates to methods for producing skeletal isomers from olefins such as normal or n-alkenes used in the petroleum and fuel industries. More particularly, this invention relates to methods for producing isobutylene from a feedstock comprised of n-butylenes.
As is known, butylene or butene exists in four isomers: butylene-1, cis-butylene-2, its stereo-isomer trans-butylene-2, and isobutylene. Conversions between the butylenes-2 is known as geometric isomerization, whereas that between butylene-1 and the butylenes-2 is known variously as position isomerization, double-bond migration, or hydrogen-shift isomerization. The aforementioned three isomers are not branched and are known collectively as normal or n-butylenes. Conversion of the n-butylenes to isobutylene, which is a branched isomer, is widely known as skeletal isomerization. The same general terminology is used when discussing skeletal isomerization of other n-alkenes and olefins, as well as paraffinic compounds such as n-alkanes.
Isobutylene has become more and more important recently as one of the main raw materials used in the production of methyl tert-butyl ether (MTBE), an environmentally-approved octane booster to which more and more refiners are turning as metallic additives are phased out of gasoline production. However, skeletal isomerization of olefins e.g., to produce isobutene, are relatively non-selective, inefficient, and short-lived because of the unsaturated nature of these compounds. On the other hand, positional and skeletal isomerization of paraffins and alkyl aromatics are fairly well established processes, in general, utilizing catalysts typically comprising metallic components and acidic components, under substantial hydrogen pressure. Since paraffins and aromatics are stable compounds, these processes are quite successful. The heavier the compounds, in fact, the less severe the operating requirements.
Olefins, however, are relatively unstable compounds. Under hydrogen pressure, they are readily saturated to the paraffinic state. Indeed, three processes could be combined for the conversion of n-alkenes to isoalkenes, for example: first, hydrogenation of olefins into paraffins; second, skeletal isomerization of the paraffins; and third and finally, dehydrogenation of the skeletal paraffins in to the desired iso-olefin. In this process combination, the first and third processes are accompanied by large heat effects and therefore may require several stages each; for light hydrocarbons, the conditions for the third process of the combination are usually quite severe.
Furthermore, in the presence of acidity, olefins can polymerize, crack and/or transfer hydrogen. Extensive polymerization would result in poor yields, and short operating cycles. Similarly, cracking would reduce yield. Hydrogen transfer would result in saturated and highly unsaturated compounds the latter being the common precursors for gum and coke. Any theoretical one step process for producing skeletal isomers of, for example, n-butylenes would have to be concerned with the unwanted production of butanes and the reverse problem of production of butadienes. On top of all of these problems, it is well known that skeletal isomerization becomes more difficult as hydrocarbons get lighter.
Representative, for example, of the above prior art isomerization efforts, Myers, U.S. Pat. No. 3,979,333, discloses a catalytic process for the skeletal isomerization of acyclic paraffins and naphthenes. The catalyst contains a Group VIII metal on alumina, which is activated by a gas comprising a mixture of different types of halides.
British Pat. No. 953,187 teaches a catalytic process for the isomerization of C.sub.4 and higher paraffins, utilizing a catalyst comprising a hydrogen-containing alumina, a Group VIII metal and halogen compounds, in which process fairly high levels of hydrogen-to-hydrocarbon ratios are employed.
Rausch, U.S. Pat. No. 3,642,925 discloses a method and catalyst for effecting both positional and skeletal isomerization of hydrocarbons including C.sub.4 -C.sub.7 paraffins and olefins. A relatively complex dual-function catalyst is employed, comprising at least five components: a zeolite-type base; a tin component; a Group VIII metal, preferably platinum; a rhenium component; and preferably a halogen component. Skeletal isomerization of butanes is exemplified.
Hayes, U.S. Pat. No. 3,919,340 discloses positional isomerization of olefins and positional and skeletal isomerization of paraffins, cycloparaffins, and alkylaromatics. Once again, a relatively complex dual-function catalyst is utilized comprising five components: a zeolite-type base carrier; a Group VIII metal; an iridium component; a germanium component; and a halogen component. It is important that the Group VIII metal and the iridium be present in elemental metallic states, and that substantially all of the germanium be present in the oxidation state.
Manara et al, U.S. Pat. No. 4,038,337 discloses a method for the skeletal isomerization of alkenes, and specifically discloses the conversion of n-butenes to other n-butenes and iso-butene. The catalyst utilized is obtained by reacting an active alumina with an ester of silicic acid, preferably the lower alkyl esters of orthosilicic acid. It has been found that the latter process generally has a short-cycle length between regenerations, sometimes as little as one day. Additionally, high temperatures are generally required, usually exceeding 450.degree. C.
Because of the increasing importance to the fuel and petroleum industries of the availability of a process which efficiently and readily produces isoalkenes from feedstocks containing n-alkenes, it is a principal object of this invention to design such a process which does not require frequent regeneration and high temperature. It is a further object of this invention to design such a process which strikes a desirable balance between the production of isoalkenes on the one hand, and the repression of the production of their corresponding carbon-number alkanes on the other hand.